1. Field of the Invention
The present invention relates to methods and apparatus using fluorescent emissions to determine a concentration of a material in an object. More specifically, the present invention relates to methods and apparatus using fluorescent emissions to determine a concentration of dopant in a soot preform used to form optical waveguide fibers ("optical fibers").
2. Description of the Related Art
An optical fiber typically includes a cladding made of pure silica (SiO.sub.2) and a core made of silica doped with germania (GeO.sub.2). The germania dopant alters the refractive index of the silica in the core. Portions of the core often contain different concentrations of germania, resulting in different refractive indexes along the diameter of the core. The distribution of refractive indexes along the diameter of the core (i.e., the refractive-index profile) determines operating characteristics of the optical fiber.
The optical fiber can be formed by a conventional process known as outside vapor deposition ("OVD"). Generally, the OVD process involves forming a soot preform by burning a gaseous mixture to produce soot containing silica and germania, successively depositing layers of the soot onto a mandrel to form a core portion of the soot preform, burning a gaseous mixture to produce soot containing only silica, and successively depositing layers of that soot onto the core portion to form a cladding portion of the soot preform. The soot preform is consolidated by sintering to form a glass blank. An optical fiber is drawn from the glass blank. The concentrations of germania in tie soot layers forming the core portion primarily determine the concentrations of germania along the diameter of the core of the resulting optical fiber.
If the concentrations of germania in the soot layers can be measured, a soot preform can be evaluated to determine whether it can be expected to produce an optical fiber with a desired refractive-index profile. Also, if dopant concentration can be determined on-line, i.e., during soot deposition, the dopant concentration can be monitored and altered to obtain a desired profile.
Japanese Patent Application No. 59-106803 (Hara) and U.S. Pat. No. 4,618,975 (Glantschnig) disclose techniques that use X-ray attenuation to nondestructively evaluate the concentrations of germania in soot preforms. Both approaches measure X-ray attenuation at two energies. Hara's scheme relies upon the fact that the dopant (Ge) to matrix (Si) attenuation ratio changes with X-ray photon energy. Hara's scheme is not particularly sensitive for soot preforms, however, because the ratio is nearly constant over any practical X-ray energy range. Glantschnig's method is based on the fact that the ratio of dopant attenuation (absorption) to density attenuation (scattering) changes with X-ray photon energy. Like Hara's ratio, Glantschnig's ratio is nearly constant over an energy range practical for soot preforms. Thus, Glantschnig's method confounds density changes with dopant concentration changes.
These X-ray attenuation methods have additional disadvantages. For example, if a soot preform has multiple dopants, X-ray attenuation due to one dopant cannot be distinguished from X-ray attenuation due to another dopant. Moreover, the measurement of attenuation requires precise location of the preform within the measurement apparatus, and therefore it is expensive to implement.
U.S. Pat. No. 4,292,341 (Marcuse) discloses methods of performing on-line measurements of dopant concentration in a modified chemical vapor deposition process, which does not form a soot preform but instead immediately consolidates the soot into a glass blank. One of the disclosed methods employs X-ray attenuation, which has many of the problems mentioned above. Another method measures dopant concentration by irradiating the glass blank with ultraviolet light and measuring the fluorescent emissions of the glass blank. It is believed that the latter method will not work with a soot preform, since soot is opaque to ultraviolet and visible light.